Double Stall

 

In 1999, Dick Veldkamp from former manufacturer NEG Micon started a project to understand the occurrence of a mysterious phenomenon called 'Double stall'. He hired Gustave Corten, because he did a PhD on stalling of wind turbines and had developed the stall flag method. 

 

Double stall was the phenomenon  that stall regulated wind turbines of e.g. 550kW sometimes suddenly had a stall level of about 400kW instead of 550kW.  The lower level could maintain for weeks and then suddenly jump back to 550kW.  At a later time it could jump back down to a lower level. This all occurred without any change in the turbine control. 

 

By using the stall flag method, analyzing many hypotheses Corten finally concluded that Double Stall was caused by the particular flying behavior of insects, in particular hat of flying ants: at the special dates (low wind, high humidity and in fertile period) that the ants fly, the blades get highly contaminated. At high wind speeds the contamination advances stall. When a shower would pass by at a later time,  the blades are 'washed' and the stall level went back to the designed level.